It would be great if specific Ship type had dimension limits, e.g. one type could be max 3 blocks wide and other 5 blocks wide.
This could be useful when you want to limit new players to smaller vessels, as they aren't so skilled in driving them and they can easily crash.

This is a good idea, however while the majority of people put the licence sign facing forward or backwards, there are some who put it right or left, therefore Ships cannot determine what is forwards, backward, left or right.

So the best I can do is length of a square cube.

I do not see that as a problem:

• If you specify one limit (e.g. 10):
  It would be the "length of square" - Ship 10x10 OK, Ship 10x15 FAIL, Ship 15x10 FAIL

• If you specify two limits (e.g. 10, 15):
  Plugin will check both combinations: Ship 10x15 OK, Ship 15x10 OK, Ship 16x9 FAIL, Ship 2x18 FAIL

I can imagine the format would be height, width, length, where -1 would be no limit:
dim_limit: -1, 10, 15 would be the limit from the second example above

Thats true actually.